Rotation of the femurs (BOTH of them ) in the hip sockets. This prevents the "dipping" of the head and/or lifting of the heel of the support leg. It also can add energy to the motion. Ideally you can "cock the trigger" by overemphasizing the sanchin toe-turn-in. This IMO is why Uechika CAN BE masters of the kick off the front leg. It's the stored energy built in the sanchin stance!!
One thing you emphasized that really stuck with me, Bill, was that rotation of the standing foot that occurs with the rotation of the femurs....makes all the difference in the world as far as that whipping action to create the power (ie. that is toeing in with the standing foot before the kick to cock the trigger, then allowing the standing foot to rotate or pivot out and back with the kick afterward with the hip snapping action.
Does that make sense in words? You are so much better describing, Bill, than I am....technical engineering man that you are.
That is my take on a noticeable transition.